Such a circuit-breaker is described in Patent Application EP-0 591 039 filed by the Applicant.
In that prior art document, the piston is semi-moving, and it is provided with means for holding the piston stationary during a first portion of the displacement of the moving contact assembly between the closed position and the open position, and means for displacing the piston axially with the moving contact assembly during a second portion of the same displacement of the moving contact assembly. The means for displacing the piston axially are constituted by a drive member secured to the moving contact assembly, which drive member drives an abutment secured to the piston during the second portion of the displacement of the moving contact assembly, the abutment being disposed in the path of the drive member. The piston is connected to a third tube provided with at least one slot in which a finger is slidably mounted, the finger being secured to the first tube and extending radially therefrom towards the second tube. The means for holding the piston stationary are constituted by a spring disposed between the drive member and the piston, and by a fixed retaining member that co-operates with the piston.
When low currents are to be interrupted, the arc that is produced between the arcing contacts during an opening step is extinguished by means of compressing the gas in the compression chamber. But the arc is blasted by a flow of gas from the compression chamber before the end of the displacement of the moving contact assembly. Therefore, it is not necessary to compress the gas in the compression chamber during the entire displacement of the moving contact assembly. Once the piston starts being displaced with the moving contact assembly, the quantity of energy required to drive the moving contact assembly is very small because the gas is no longer compressed.
Unfortunately, in that known configuration, the spring urging the semi-moving piston continuously applies a force to the control rod, thereby giving rise to an increase in the drive energy required.